Update naming function

Summary: Update naming function to always add "cfg:" in front of configuration names generated by modifiers

Reviewed By: stepancheg

Differential Revision: D51341821

fbshipit-source-id: 0596b245de8a5dc9d642d7bb9e5f62d5265f0094

cfg/experimental/name.bzl

@@ -16,6 +16,12 @@ NAMED_CONSTRAINT_SETTINGS = [
     "ovr_config//build_mode/constraints:san",
 ]
 
+# Mark all modifier generated configurations with a `cfg:` prefix.
+# We do this so that we can easily recognize which configuration is generated
+# by modifiers and query for it in Scuba.
+_CFG_PREFIX = "cfg:"
+_EMPTY_CFG_NAME = _CFG_PREFIX + "<empty>"
+
 def cfg_name(cfg: ConfigurationInfo) -> str:
     """Derives a reasonable name for a ConfigurationInfo"""
 
@@ -24,4 +30,8 @@ def cfg_name(cfg: ConfigurationInfo) -> str:
     for constraint_setting in NAMED_CONSTRAINT_SETTINGS:
         if constraint_setting in constraints:
             name_list.append(constraints[constraint_setting].label.name)
-    return "-".join(name_list)
+    if len(name_list) == 0:
+        name = _EMPTY_CFG_NAME
+    else:
+        name = _CFG_PREFIX + "-".join(name_list)
+    return name